How to set default pose to Unity of model from exported as FBX from Blender?
You should Know that Unity takes the "default pose" from the first frame of the first animation.
Also Unity classifies them as they are in blender:



[Solution]
1 - Always create an Idle Animation and set the "default pose" in the first frame, and rename it to be the first animation.

or

2 - set "default pose" on the first frame of your first animation(action) in blender, so Unity can understand that's the default pose.

(don't forgit to Reimport the .FBX in Unity after making changes)